This invention relates to a process for the production of low molecular weight silanol-stopped materials by reacting hexaorganocyclotrisiloxanes with water in the presence of catalytic amounts of a strong base. The reaction is preferably run in a water/acetone system.
Currently, silanol stopped oligomeric siloxane materials are made by two processes. The first process involves controlled hydrolysis of dichlorodimethylsilane in a water/polar solvent mixture in the presence of a neutralizing agent, such as sodium bicarbonate or ammonia. This process produces a significant amount of waste and about 30% of cyclic oligomers which have to be removed from the final product by vacuum stripping.
The second process involves ring opening polymerization of hexamethylcyclotrisiloxane in a water/ organic solvent mixture in the presence of acid-activated clay, (such as FILTROL-20) as disclosed in U.S. Pat. No. 3,853,932, or the acidic or basic ion exchange resins as described in for instance, U.S. Pat. No. 3,309,390. The process requires filtration of the catalyst and high vacuum distillation to remove cyclic oligomers formed during the ring opening polymerization.
Broadly conceived the present invention provides for a process for the production of linear silanol stopped siloxanes comprising:
a) conducting a ring opening polymerization of a hexaorganocyclotrisiloxane having the formula:
D3=(R1R2SiO)3
where R1 and R2 are independently selected from the group of one to forty carbon atom monovalent radicals, in a solvent comprising a mixture of water and a volatile polar, aprotic organic solvent in the presence of catalytic amounts of a strong base;
b) neutralizing the catalytic amount of the strong base with a partially neutralized salt of a polybasic acid wherein the pH ranges from about 6 to about 8; and optionally
c) washing with water to remove salts, or, altenatively, stripping water and acetone from the batch and filtering the neutral salts.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein.
Before the present compositions of matter and methods are disclosed and described, it is to be understood that this invention is not intended to be limited to specific synthetic methods or to particular formulations, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.
In the following specification, reference will be made to a number of terms that shall be defined to have the following meanings:
The singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise.
xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
It has been discovered that a ring opening polymerization of hexamethylcyclotrisiloxane, D3, in water/ volatile polar, aprotic organic solvent mixture in the presence of catalytic amounts of a strong base such as LiOH, NaOH, KOH, or a mixture thereof proceeds with a high rate to afford linear oligomeric silanol-stopped siloxanes with a high yield, typically above about 90%. The volatile polar organic solvent used with water to form the solvent of the process must be miscible with water. Non-limiting examples of such volatile polar aprotic organic solvents are low molecular weight ketones, e.g. acetone. The amount of cyclic oligomer by-product, such as octamethylcyclotetrasiloxane, D4, is very low. As used herein D3, also referred to as hexaorganocyclotrisiloxane, is defined as follows:
D3=(R1R2SiO)3
where R1 and R2 are independently selected from the group of one to forty carbon atom monovalent radicals where such radicals may or may not be substituted by other substituents, e.g. halogen atoms. In addition to the alkyl aralkyl and aryl groups subtended by the phrase xe2x80x9cone to forty carbon atom monovalent radicals,xe2x80x9d this definition specifically includes, without limitation, fluoro substituted radicals such as 3,3,3-trfluoropropyl as well as perhalo-alkyl radicals and perhalo-aryl radicals.
After neutralization of the strong base with weak acids, such as carbon dioxide or acidic salts, and a subsequent water wash, the system is stable. The removal of the volatile polar aprotic organic solvent, e.g.acetone, by a simple distillation yields the desired silanol-stopped organosiloxanes with high yield. It was unexpectedly found that the process does not require filtration (because no solis are present after a water wash), and high vacuum distillation (since only small amounts of non functional cyclic siloxanes, such as octamethylcyclotetrasiloxane are formed).
With respect to neutralization, the catalyst must be neutralized to prevent condensation of the silanols during removal of solvents. It is preferable to be close to a pH 7 after neutralization of the basic catalyst preferably the pH range after neutralization is from about 6.7 to about 7.3, more preferably from about 6.5 to about 7.5 and most preferably from about 6.0 to about 8.0. This may be accomplished by measuring the base content of the reaction and precisely measuring any acid, including hydrochloric, sulfuric, acetic, etc. However, such an approach requires strict control of the concentration of acids, because if any slight excess or absence of such acids from exact stoichiometry would result in a pH which is too high or too low and cause loss of silanol during stripping. Useful deactivating agents include carbon dioxide, and acidic salts of polybasic acids. Under some conditions even carbon dioxide can produce deactivated salts which are too high in pH. As used herein polybasic acids are acids containing more than one neutralizable acid functionality per molecule, which when partially neutralized function as a buffering agent for control of pH. Therefore, most preferred are salts whose aqueous solution are close to pH 7 and which will react with the basic catalyst to produce deactivation salts which are also in the range of pH 6 to pH 8. Preferred are the monobasic salts of di- or tri-basic acids (a subset of polybasic acids). Such salts would include the partially neutralized alkali metal salts of such polybasic acids, e.g. sodium or potassium hydrogen sulfate, mono sodium or potassium oxalate and other dibasic organic acids, e.g. mono sodium or potassium dihydrogenphosphate. Most preferred is the mono sodium or potassium dihydrogenphosphate. As used herein the term alkali metal includes lithium, sodium, potassium, rubidium and cesium. By varying the molar ratio of this salt to the catalyst, the pH of the deactivated catalyst can easily be maintained near pH 7. Even when the amount of sodium dihydrogenphosphate is in excess or shortfall, the pH will remain near pH 7. The most preferred ratio is 2 moles of sodium dihydrogenphosphate per mole of basic catalyst.
The process of the present invention produces linear slioxanes in excess of 90% yield and preferably in excess of 90% yield, more preferably in excess of 95% yield and most preferably in excess of 97% yield.
The process of the present invention allows the production of linear silanol-stopped oligomers with a higher yield than known processes. The process of the present invention provides a faster rate of reaction, and the filtration step required in the prior art is eliminated by the use of a homogenous catalyst system. The process of the present invention produces less cyclic byproduct than the prior art, thereby eliminating the need for a high vacuum distillation.